Chromium surfaces are used in various applications such as a decorative metal finish for plastic parts in automotive and sanitary industries or as wear resistant coatings for plated parts such as shock absorbers. The chromium surface is usually the outer surface of the substrate and obtained by electroplating a chromium layer from plating bath compositions comprising either Cr(III) ions, Cr(VI) ions or both.
The resulting chromium surface is usually very shiny and fulfils aesthetic requirements. The corrosion protection provided by the chromium layer to the underlying substrate is usually increased. However, in some applications of chromium surfaces such as in the automotive industry, the corrosion protection provided by the chromium layer is not sufficient, e.g. in case when 480 h ISO 9227 NSS-test without change of appearance of the chromium surface is required. This requirement can at the moment only be fulfilled by application of post-treatment methods with solutions comprising toxic Cr(VI) ions.
At least one other metal or metal alloy layer is located between said chromium layer and the substrate. The at least one metal or metal alloy layer is selected from one or more of nickel layer, nickel alloy layer, copper layer and copper alloy layer.
The chromium layer usually comprises micro-cracks after plating or (thermal) annealing, or pores created by an underlying micro-porous nickel layer. Hence, also the layer material(s) between the chromium layer and the substrate are exposed to the environment. Accordingly, the undesired corrosion of substrates having a chromium layer as the outer surface is caused by the corrosion of the underlying layers. The chromium oxide layer formed on the outer surface of the chromium layer protects said outer surface of the chromium layer from corrosion but not the underlying layer(s). Such multilayer assemblies comprising a chromium layer as the outermost layer are for example disclosed in US 2012/0052319 A1.
Different methods to increase the resistance to corrosion of chromium surfaces and the underlying metal and/or metal alloy layer(s) are known in the art.
Coating agents comprising polymers which contain 0.05 to 3 wt.-% sulfonate and/or phosphonate groups or their respective esters applied for cathodic electrocoating of electrically conductive substrates are disclosed in U.S. Pat. No. 4,724,244. Said polymer is deposited onto the electrically conductive substrate and thereby forms a corrosion protection layer having a thickness of several μm such as 18 μm. The resistance of corrosion is increased by said treatment but the optical appearance of a chromium surface and the surface feel is drastically changed by the thick polymer layer which is not acceptable for e.g. decorative applications of the chromium surface. Furthermore, this method requires a thermal curing of the as deposited polymer which is, due to the necessary high curing temperatures, not applicable to plastic substrates common in automotive industries.
An anodic treatment of metal surfaces with an aqueous solution comprising a compound having hydrophobic carbon-chains with hydrophilic anionic functional groups is disclosed in EP 2 186 928 A1. The resistance to corrosion can be increased by said method but residues creating a foggy appearance remain on the metal surface even after rinsing with water, especially on dark chromium surfaces. Hence, said method is not suitable to increase the resistance to corrosion of a chromium surface and maintain the optical properties of said chromium surface, i.e. the shiny and decorative optical appearance.